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Session 33 - Comets & Asteroids.
Oral session, Wednesday, January 07

[33.04] Properties of cometary comae, deduced with ISOPHOT

S. B. Peschke, E. Grün (MPI für Kernphysik), ISOPHOT Comet Team

The Infrared Space Observatory (ISO) provided a unique opportunity to study the thermal dust emission from cometary comae. We used the spectrophotometer ISOPHOT for photometric measurements on the comets 22P/Kopff, 81P/Wild, 29P/Schwassmann-Wachmann 1, 2060 Chiron and C/1995 O1 (Hale-Bopp). Multi-filter and multi-aperture photometry between 3.6-160\mu m were combined as an observational sequences for these five comets. The observations cover a range from 1.7AU out to 8.4AU in distance. Almost always (quasi-) simultaneous ground-based observations in the optical and near/mid infrared were coordinated. These data sets cover the whole cometary coma spectrum, the scattered light being covered by the visible, the thermal dust emission accessible through the ISO observations and the near/mid infrared offering a closer look where both components join. After very complicated data reduction and despite calibration uncertainties, we were able to determine temperature estimates for the dust in the coma by fitting a simple black body to the data. The temperatures range from about 100 K for Chiron to ca. 220 K for C/1995 O1 (Hale-Bopp). Extensive grain size distribution modeling was applied to the ISOPHOT data. Three different grain materials for the coma were tested, silicates, carbonaceous material and water ice. Twelve different grain size distributions likely to be found in comets were applied. The properties of the grains itself were determined with Mie calculations. Both, single and two constituent models were compared to the data, \chi^2 values being a measure for their agreement with the data. For the distant objects the cometary comae were most likely composed of small silicate grains with an icy component added. The ISOPHOT data for more active comets at smaller heliocentric distances can be best explained by cometary comae dominated by carbonaceous grains. A change in the size distribution from pre- to post perihelion for 22P/Kopff as well as for C/1995 O1 (Hale-Bopp) in and outside of 3AU was found.

Program listing for Wednesday